Biodiesel, which is a renewable fuel that has similar combustion properties to fossil diesel, is normally produced by the transesterification of highly refined oils with short-chain alcohols. Since biodiesel can significantly decrease the emission of CO2, SOx, and unburned hydrocarbons from the exhaust of motor vehicles, it is environmentally beneficial, and therefore, a promising alternative to fossil diesel.
Biodiesel is typically produced in a conventional manufacturing operation via the catalyzed esterification and transesterification of free fatty acids (FFA) and triglycerides, respectively. In this reaction the oil or fat is reacted with an alcohol to form the biodiesel and glycerine. Such a conventional operation will usually use a strong basic (e.g., NaOH or KOH) or acidic (e.g., H2SO4) solution as a homogeneous catalyst and highly refined oils as the feedstock. These homogeneous catalysts are quite sensitive to the presence of FFA and water in the oil feedstock. FFA is known to react with basic catalysts (i.e., NaOH or KOH) to form soaps. The formation of soaps will subsequently complicate the separation of glycerol from the reaction mixture, thereby, drastically reducing the yield of fatty acid methyl esters (FAME).
The price of the oil feedstock is a dominating economic factor associated with the manufacturing of biodiesel products. The cost associated with using a highly refined oil feedstock is high due to the demand for these oils in multiple industries, such as the food, cosmetics, and livestock feed industries. In fact, the cost of the oil feedstock can account for up to a total of 80% of the cost to produce biodiesel. In order to reduce the cost associated with forming biodiesel products, other less expensive feedstocks, such as waste oils (e.g. yellow grease and brown grease) and acidulated soaps will need to be utilized.
Waste oils are different from refined or crude oils by the presence of free fatty acids (FFAs) that are generated through the hydrolysis of triglycerides during a typical cooking process. For example, yellow grease is typically used-frying oils recovered from deep fryers, whereas brown grease is the grease that is recovered from a waste water component called a grease trap. The FFA content in the oil directly affects the price of the feedstock, e.g., the FFA level in yellow grease at <15% and in brown grease at >15% results in the brown grease being less expensive than yellow grease.
The ability to use a variety of less expensive, lower grade oils or lipids would make biodiesel much more competitive with petroleum-based fuels. Accordingly there exists a continual need or desire to develop new catalysts for use in biodiesel production that will allow such lower grade oils to be utilized.